Signal transduction proteins have increased importance in carcinogenesis and tumor formation and represent attractive targets for the development of novel anticancer therapeutics.
The Signal Transducer and Activator of Transcription (STAT) family of proteins are cytoplasmic transcription factors with important roles in mediating responses to cytokines and growth factors, including promoting cell growth and differentiation, and inflammation and immune responses (1,2). Normal STATs activation is initiated by the phosphorylation of a critical tyrosine residue upon the binding of cytokines or growth factors to their cognate receptors. The phosphorylation is induced by growth factor receptor tyrosine kinases, or cytoplasmic tyrosine kinases, including Janus kinases or the Src family kinases. While pre-existing dimers have been detected (3,4), phosphorylation is observed to induce dimerization between two STAT monomers through a phosphotyrosine interaction with the SH2 domain. In the nucleus, active STAT dimers bind to specific DNA-response elements in the promoters of target genes and regulate gene expression.
It is now well established that aberrant activation of the member of the family, Stat3 contributes to malignant transformation and tumorigenesis. Aberrant Stat3-mediated oncogenesis and tumor formation is due in part to the transcriptional upregulation of critical genes, which in turn lead to dysregulation of cell growth and survival, and the promotion of angiogenesis (2,5-11) and tumor immune-tolerance (12,13). Thus, targeting of aberrant Stat3 signaling would provide a novel strategy for treating the wide variety of human tumors that harbor abnormal Stat3 activity.
The critical step of dimerization (14) between two monomers within the context of STAT activation presents an attractive strategy to interfere with Stat3 activation and functions and this approach has been exploited in prior work (15-25). Leading agents from those earlier studies have been explored for rational design in conjunction with molecular modeling of the binding to the Stat3 SH2 domain (18,19), per the X-ray crystal structure of the Stat3 homodimer (26). One of those leads, S3I-201 (18) had previously been shown to exert antitumor effects against human breast cancer xenografts via mechanisms that involve the inhibition of aberrant Stat3.
In the present study, key structural information from the computational modeling of S3I-201 bound to the Stat3 SH2 domain facilitated the design of novel analogs of which S3I-201.1066 and S3-201.2096 show improved Stat3-inhibitory activity. Both S3I-210.1066 and S3I-201.2096 inhibit Stat3 activity with IC50 values of 35 and 45 μM, respectively. This disclosure presents evidence that S3I-201.1066 interacts with the Stat3 protein and disrupts Stat3 binding to its cognate pTyr peptide of receptors. Furthermore, S3I-201.1066 induces antitumor cell effects selectively in malignant cells harboring aberrant Stat3 and antitumor response in vivo in human breast xenografts.